C3 Corvette Engine Build - Welcome To The Machine (Shop)

Building A Small-Block Chevy To Out-Power The LS7

For a VETTE Magazine project car to be true to the name, it has to perform-and no piece of the puzzle is more critical to performance than the powerplant. If you've been following our project, then you know that we've been modifying every aspect of Project C3 Triple-Ex, from the steering and suspension to the brakes and transmission. More recently, we upgraded the interior with replacement panels, race-inspired seating, four-point harnesses, and full Auto Meter instrumentation. Now that the car is finally ready, the culmination of our project is to build an appropriate engine.

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Last month we told you of our plans, brought on in part by Editor Heath's challenge to build a first-generation small-block Chevy engine that could out-power the formidable late-model LS7. While the LS engine does have some design advantages over the old small-block, the SBC is backed by a wealth of racing technology developed over the past 50-odd years. We're confident that these tips and tricks will enable us to exceed the LS7s power rating with a small-block of identical 427ci displacement.

While the temptation of an LS swap was certainly attractive, building an SBC for our project vehicle proved the better option for several reasons. Since our '71 is already equipped with a warmed-over 350, installing an architecturally similar engine will be simpler than adapting the engine bay for a late-model replacement. The expenses will be lower as well, as we won't need custom motor mounts, headers, or flywheel and bellhousing adapters to make the engine fit. Additionally, far fewer fuel-system modifications will be required for a carbureted small-block, as compared with a fuel-injected LS powerplant. With these factors in mind, we laid out a plan for our engine.

In last month's issue we highlighted the parts we'll be using for this build, including a Dart aluminum block, Dart Pro1 CNC cylinder heads, and a lightweight forged rotating assembly utilizing parts from Mahle and Summit Racing Equipment. We'll be duplicating the LS7 in terms of both bore and stroke, for an identical displacement. And thanks to its all-aluminum construction, our SBC should weigh roughly the same as well. But to actually out-power the LS7, we'll need to employ some engine-building tricks, many of which will be accomplished during the machining phase of the build.

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This month we'll visit our local machine shop, Auto Performance Engines in Auburndale, Florida, and follow along as owner Kevin Willis and his staff perform the necessary operations. The machining process isn't fully understood by many enthusiasts, but this phase is critical to an engine's durability as well as its output. Inaccurate machine work can lead to disappointing power numbers, as rings have trouble sealing to cylinders that aren't perfectly round and true. In fact there are a number of machining operations that can make or break an engine, and we'll show you all of them in this article.

We often hear that one machine shop charges significantly more than another for "the same work." But while the operations performed might be the same, the quality of the work isn't. It takes time to perform machine work accurately and correctly, and tooling must be replaced regularly to maintain this standard of accuracy. If we compare two identical engines-one with poorly performed machine work, and the other with accurately performed work-they may make similar power initially. After some street use or a few trips to the track, however, the "budget" engine will often make itself apparent by running slower lap or elapsed times, making less power on a dynamometer, burning oil, or, worse yet, scattering its internals through the oil pan or block.

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C3 Corvette Engine Build - Welcome To The Machine (Shop)

Last month we outlined the parts we'll use to build an LS7-killer small-block for Project C3 Triple-Ex. This month we'll take you to the machine shop and show you the process, along with a few tricks we'll utilize to achieve our goal.

The Dart aluminum block we're using isn't designed to be a finished product, since the level of machining greatly depends on intended use and the other parts of the engine. We'll need to perform several procedures before the block will be ready for the build.

Dart provides extra material on the deck of the block, allowing it to be machined to the proper height for the application. We wanted our Mahle dished pistons right at the top, which required installing the crank, rods, and pistons, then physically measuring how much material needed to be removed.

Auto Performance Engines (APE) installs the main caps and uses torque plates to simulate the presence of cylinder heads during the cylinder-machining process. This ensures that the block is under the same stresses it will experience once it's assembled and running. We consider this the only way to properly size cylinders in a performance engine, so if your shop doesn't use torque plates, you might want to go elsewhere.

Another important consideration is the roughness, or grit, of the hone. This is determined by the type of piston ring that will be used. APE has lots of hones for many applications, and chose the ideal one for our application. This will ensure that our rings seal precisely to the cylinder, creating a good seal with minimum friction.

With the engine mocked up, the bottom of the cylinders can be marked for connecting-rod clearance. This step is necessary since we're increasing the stroke of our small-block to 4 inches with a Scat lightweight forged crankshaft from Summit Racing Equipment.

The Dart block is designed for either wet- or dry-sump oil systems. Since we'll be running a wet-sump system, we had to machine provisions for the oil to drain back to the pan. This was accomplished by notching the valley, then drilling holes in the rear of the valley, away from the majority of the rotating assembly.

We'll be using a Jesel beltdrive system to drive our camshaft...

...so we needed to perform a couple of minor modifications to the front of the block for clearance. The beltdrive offers many advantages over a timing chain, which you'll see on the dyno. This is a definite advantage over the LS engine, as the oil pump of the LS makes using a beltdrive system impossible.

You can tell a lot about a machine shop by how clean it's kept, and the APE shop is as clean as they come. Once the machining processes on the Dart block were complete, all the pieces were thoroughly cleaned and prepped for assembly.

We ordered our block with the option of 50mm roller cam bearings. This allowed us to take advantage of the larger-diameter camshaft core, eliminating torsional flex of the cam for more-accurate valve timing. The camshaft is also in a raised location, so we can use shorter pushrods for less flex.

The roller bearings use a special installation tool, which APE has in stock.

We needed to mill the cylinder heads to achieve the desired 11:1 compression ratio, but that caused piston-to-valve clearance problems. In order to correct this anomaly, we fly-cut deeper valve reliefs into our Mahle forged pistons.

Milling the cylinder head reduces the combustion-chamber size, increasing compression. We couldn't achieve the desired compression ratio by flat-milling
the heads, but fortunately Dart leaves plenty of meat on the deck for angle-milling. Only a dedicated performance machine shop like APE can perform this procedure properly, so be careful when choosing a shop to do this type of work.

The Dart cylinder heads we're using come assembled and ready to install, but since we had to angle-mill the heads, complete disassembly was required. While everything is torn down, we'll perform a few tricks to ensure this engine makes great power and lives a long life.

Dart provides the complete specs for the valvesprings installed on the Pro1-CNC heads, but we wanted to verify for ourselves that there would't be any coil-bind issues-especially since we're using a fairly aggressive Comp solid-roller cam. The springs checked out just as Dart advertises, and will work perfectly with our combination.

Here's a trick you won't see many shops performing. Since the steel valvesprings can mar the titanium spring retainers, APE uses a conical grinder to chamfer each spring, eliminating sharp edges.

The Dart Pro1-CNC heads are ported and flow tested, and Dart publishes the flow numbers-so why are we flowing our heads? Since each flow bench is a little different, we wanted to get numbers on the APE bench as well. We'll use the information to evaluate our combination once the engine is dyno'd, and as a point of reference for future projects. As it turned out, the Dart ratings were fairly conservative-the heads flowed even more than advertised on APE's bench.

To ensure correct bearing clearances, the main caps are installed and tightened to the proper torque, then checked for alignment. Each bearing is then measured and matched to the crankshaft journals to achieve proper clearance. We found our Dart block and Scat crank to be properly machined from the factory, so no additional machine work was required to achieve the right oil clearances.

The same procedure is used to attain the proper connecting-rod oil clearances: Each journal is measured both on the crankshaft and on each connecting rod, then bearings are mixed and matched to gain the right clearance.

The Scat rods we got from Summit Racing were right on the money, requiring a minimum of work.

With critical dimensions verified, all of the parts are cleaned again before the engine is assembled for final clearance checks and pushrod-length measurements.

Pushrod length is one of the final dimensions determined before assembling the engine, as it depends on many variables such as deck height, the amount milled from the heads, lifter and camshaft design, and the thickness of the compressed head gasket. We installed our heads with pre-compressed head gaskets, then bolted on our Jesel rockers and used adjustable "checker" pushrods to come up with a length of 7.5 inches.

As a final step, APE will balance our rotating assembly, and then we'll be ready for final assembly. Next month we'll bolt our LS7-killer all-aluminum small-block together, then strap it to the dyno to see if we've achieved our goals!

After that, our 427 will be installed in Project C3 Triple-Ex for some real-world street and track testing.

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